1. Field of the Invention (Technical Field)
The present invention relates to methods for separating solids from liquids, particularly to a method of desalinating water.
2. Background Art
Conventional methods of thermal desalination typically use distillation of seawater through a process of evaporating seawater and condensing the resulting water vapor, producing xe2x80x9csalt-freexe2x80x9d water. The most frequently encountered method of thermal desalination, commonly called xe2x80x9cmulti-stage flash,xe2x80x9d requires high temperatures, and operates from ambient (or elevated) pressures in the first stage to a high vacuum in the last stage, to effect temperature and pressure gradients for distillation and efficient heat utilization. The method produces desalinized product at only about 35% efficiency, meaning that only 35 gallons of distilled water results from every 100 gallons of input seawater. The multiple stages, pressures, vacuums, and low efficiency are economical barriers to wider use of the technology.
Other known desalinization processes involve the use of expensive reverse osmosis membranes to separate dissolved solids from water. The practicality, and popularity, of reverse osmosis membrane systems are hindered by the capital costs of RO membranes, as well as expense associated with RO membrane maintenance.
Representative patents from the desalinization field and serving as useful background to the present invention include U.S. Pat. No. 3,163,587 to Champe, U.S. Pat. No. 3,243,359 to Schmidt, U.S. Pat. No. 4,200,497 to Rhodes, and U.S. Pat. No. 3,642,393 to Ross et al. More recent disclosures in the field include U.S. Pat. No. 5,207,928 to Lerner, and U.S. Pat. No. 4,323,424 to Secunda et al.
A number of patents describe the separation of dissolved solids from liquids through the use of creating small droplets of the solution by rapid passage of the solution through a pneumatic nozzle, i.e., a nozzle which drives the solution spray primarily with a jet of compressed air blown into, or mixed with, a solution stream. The known pneumatic-nozzle methods typically atomize the solution in a chamber at ambient temperature and pressure. Although such methods can separate salt (sodium chloride, as well as other chemical salts such as potassium chloride) from seawater, large production rates have not been achieved or predicted at economical advantages. At ambient temperature, a large stream of droplets requires extremely large volumes of air and large chambers to effect complete evaporation. The prior apparatuses do not describe economical methods, and they employ methods of condensing that are economically unattractive, being costly to scale up to commercial levels of production. Furthermore, and importantly, creating droplets using compressed air requires excessive amounts of energy relative to other known methods of desalination.
It is known that atomizing the water into micron-size droplets significantly enhances their rate of evaporation. This method of solvent evaporation and dehydration of solids is often referred to as xe2x80x9cfoggingxe2x80x9d when producing droplets less than 20 microns in diameter, and xe2x80x9cmistingxe2x80x9d when using larger droplets. To the applicants"" knowledge, however, none of the known methods or devices utilize fogging or misting, together with both waste heat over a range of temperatures and water collection upon cold-water condensers, to produce desalination under conditions that are economically attractive for commercial production of large quantities of fresh water from seawater.
A need remains for a desalination method that can accommodate a range of temperatures including lower temperatures such as those available from waste heat, is simple to construct and operate, does not require either large vacuum or pressure vessels, and yet can produce large volumes of product at high efficiency through the utilization of rapid evaporation and equilibration of droplets with vapor. Against this background, the present invention was developed.
Broadly characterized, the present invention is of a method and apparatus for performing economical desalination, or the separation of water from dissolved solids, using available waste heat over a broad range of temperatures. The method includes the production of small droplets using hydraulic pressurization, the subsequent rapid evaporation of those droplets in a flow of air which has been heated through heat exchange with a waste heat source, followed by physical separation of the precipitating salt particles from the water vapor, and the condensing of the salt-free or nearly salt-free water vapor using a metal condenser cooled by the incoming seawater. The apparatus can be scaled up to appropriate sizes for achieving commercially desired production rates at economical costs. By the invention, recovery percentages, namely the percentage of processed feed water converted into fresh water, can exceed 90%. The invention does not require multiple boiler-condenser stages, vacuum production, the use of compressed air for atomizing, or specific high temperatures. Rather, it functions at low temperatures relative to conventional thermal methods of desalination and, consequently, should be less prone to scaling, fouling and corrosion.
The invention includes a method for removing and collecting water from dissolved salts in an aqueous solution comprising the steps of forcing the aqueous solution by hydraulic pressure through at least one non-pneumatic nozzle to produce droplets of aqueous solution, blowing a heated air stream through an evaporation chamber, dispersing the droplets into the heated air stream, permitting water in the droplets to evaporate, thereby separating water vapor from salt crystals in the heated air stream, filtering the heated air stream to remove the salt therefrom, and cooling the heated air stream to condense the water vapor. The step of forcing the aqueous solution preferably comprises pressurizing the solution to at least 400 psi. Also, the step of forcing the aqueous solution preferably comprises pumping the aqueous solution through a non-pneumatic nozzle orifice having a diameter of between about 0.006 inches and about 0.02 inches. xe2x80x9cForcing the aqueous solutionxe2x80x9d preferably comprises producing droplets of aqueous solution: having diameters less than about 100 microns, or more preferably comprises producing droplets of aqueous solution having diameters less than about 40 microns. The step of blowing hot air comprises blowing air having a temperature between approximately 180xc2x0 F. and about 1000xc2x0 F. The inventive method preferably further comprising the steps of collecting solid salt particles upon a filter, and periodically rinsing the filter to remove the salt.
Alternatively, the inventive desalinization process comprising the steps of pressurizing a saline solution preferably to between about 400 psi and about 1300 psi to force the solution through at least one non-pneumatic nozzle to produce solution droplets, blowing a heated air stream through an evaporation chamber, dispersing the droplets into the heated air stream, permitting water in the droplets to evaporate, thereby separating water vapor from salt in the heated air stream, filtering the heated air stream to remove the salt crystals therefrom, and cooling the heated air stream to condense the water vapor. In alternative embodiments, the solution may be pressurized to up to 3000 psi, or more preferably 2000 psi, using specially configured nozzles.
The step of pressurizing the solution comprises pumping the solution through a non-pneumatic nozzle orifice having a diameter of less than about 0.02 inches, preferably producing droplets of aqueous solution having diameters between about 1 micron and about 100 microns. The step of blowing hot air comprises blowing air having a temperature between approximately 180xc2x0 F. and about 1000xc2x0 F.
A primary object of the present invention is to remove salt and other chemical solids from a saline aqueous solution.
A primary advantage of the present invention is that, using waste heat, it desalinates water to potable quality at improved efficiencies of energy utilization, as compared to other thermal methods of desalination.